1. Field of the Invention
The present invention relates to a plunger driving structure used in a fuel pump of a diesel engine, an oil pump of a brake system and the like.
2. Description of the Background Art
Conventionally, an oil pump used in a car brake system and the like has been disclosed in Laid-open Japanese utility model publication No. 5-83372. As shown in FIG. 9, an oil pump 1 according to the document comprises a rotation shaft 2 having an eccentric part 2a, a ball bearing 3 supporting the eccentric part 2a of the rotation shaft 2, a tappet 4 arranged on the ball bearing 3 in a radial manner, and a plunger 5 arranged on the tappet 4 and reciprocated by the rotation of the rotation shaft 22.
In addition, as shown in FIG. 10, the ball bearing 3 comprises an inner ring 3a, an outer ring 3b, a plurality of balls 3c arranged between the inner ring 3a and the outer ring 3b, a retainer 3d retaining the balls 3, and a seal 3e arranged at both ends of the bearing to seal the inner space of the bearing.
The above oil pump 1 inhales and pressure feeds an oil while the plunger 5 is moved vertically by the rotation of the rotation shaft 2.
In addition, in the above document, it is pointed out that the eccentric part 2a of the rotation shaft 2 becomes unbalanced in its driven state, causing an oscillation and the like to damage the bearing and the output shaft of a motor and the like and to raise the operation sound of a transmission pump, as problems.
Thus, in order to solve the above problems, a balancer 6 having a large diameter part 6a and a small diameter part 6b is used as shown in FIG. 11. More specifically, when the large diameter part 6a is arranged at both ends of the eccentric part 2a so as to face the direction opposite to the eccentric direction, the dynamic unbalance while the rotation shaft is driven can be corrected by using a difference in centrifugal force between the large diameter part 6a and the small diameter part 6b. 
When the distance between the eccentric part 2a and the balancer 6 is large in the above plunger driving structure, since an oscillation could be generated at the time of driving, the distance between the ball bearing 3 and the balancer 6 is 0.3 mm to 0.47 mm in general, which is very small.
Although it is no problem in the bearing such as the ball bearing 3 in which grease is enclosed in the space in the bearing sealed by the seal 3e, the balancer 6 could prevent the lubricant from flowing into the bearing in the bearing that requires the lubricant to be supplied from the outside.
Meanwhile, as the miniaturization of the oil pump is increasingly demanded recently, it is considered that a needle roller bearing that is a small in thickness in the diameter direction and the like is used instead of the ball bearing 3. According to the needle roller bearing, however, the lubricant is to be supplied from the outside in general and the distance formed between track rings is small, so that it is inevitable that the lubricant supply is insufficient due to the balancer 6.
In addition, although the rotation shaft 2, the inner ring 3a and the balancer 6 are integrally rotated in the above plunger driving structure, since the outer ring 3b is fixed, friction resistance is generated at the contact part between the wall surface of the balancer 6 and the end surface of the outer ring 3b. This friction resistance could cause an abnormal noise or oscillation while the oil pump 11 is driven.
Furthermore, surface finish such as grinding is not performed on the wall surface of the balancer 6. Meanwhile, grinding is performed on the end surface of the outer ring 3b to be a reference surface in an early stage, but even when it is scratched at a subsequent processing step, it is left as it is. As a result, the contact surfaces are rough and it is considered that this roughness causes an increase in friction resistance.